Canada ThistleCirsium arvense

Canada ThistleCirsium arvense also called Creeping Thistle, is a noxious weed found throughout the United States and Canada.

It
spreads throughout lawns and landscape beds by an expanding network of
roots. These roots sprout new plants. A very persistent and hard to
control weed once established. Featuring sharp spines that make it very
unpleasant in lawns.

This noxious weed will grow and thrive in many inhospitable areas - however it prefers moist, loam soils and full sun.

Characteristics:

Deep-rooting perennial that lives several years

Spreads by rhizomes and seed

2 to 4 feet tall

Numerous flower heads about 3/4" in diameter

Flowers June through October

A rosette-type growth occurs when mowed

Spiny and serrated leaves

Leaves are attached alternate along the stem

Stems and flower bracts are spineless unlike Bull thistle

Canada Thistle Weed ID and Lawn Weed Control

Weed Identification

Flowers - Small lavender, rose-purple, sometimes white flowers
are 1/4 to 3/4 inch wide, in groups of 2 to 5 at the end of stems.
Flowers appear in June through October.

Roots - The
vigorous roots will grow 3 feet wide and 3 feet deep. The roots survive
the winter (overwinter) and produce new plants in the spring.

Leaves -
are attached alternate along the stem. They are oblong to lance-shaped
with edges that are irregularly lobed with spiny margins. Dark green on
top, light green and hairy beneath.

Fruits and seeds -
Mature seeds are brown, 3/16 inch long, and similar in shape to a chile
pepper. A plume of tannish silky hairs is attached to one end.

Weed Control

Since Canada thistle reproduces by seeds and spreads through its
creeping roots - the best way to prevent an infestation is to keep your
lawn healthy, dense and vigorous.

Hand pulling or mowing does not work because it will grow again from it roots.

Chemical control -
The most effective way to kill perennial thistle is with a systemic
herbicide that kills both roots and shoots. Herbicides are most
effective when weeds are actively growing and air temperatures are
between 60° F and 85° F.

The best time to spray with a systemic herbicide - According to Purdue University Department of Botany and Plant Pathology - is
in early June after the first flower buds are formed and before the
first flowers open and/or on fall regrowth during September and early
October.

Glyphosate (Roundup) is the best chemical to use on
Canada thistle, however it is not selective and will kill grass plants
as well as weeds. Glyphosate can be used in landscape beds and will not
harm nearby plants as long as the spray does not contact the plants you
don't want to kill.

To make Glyphosate even more effective, use
a spreader/sticker in your spray mix. Crushing or mowing leaves also
helps break the waxy leaf layer allowing the chemical to penetrate into
the leaves. Another option, use a paint brush to "paint" straight
Roundup - not mixed with water - directly onto the thistle leaves.

Broadleaf
herbicides are most effective when plants are young and actively
growing. Mowing to generate growth before spraying works well.
Herbicides containing triclopyr or dicamba (Trimec products) are more
effective than those commonly used containing 2,4-D or MCPP.